In the preparation of printed circuit boards for the electronics industry, it has become commonplace to provide multi-layer boards, two-sided boards, or the like, with through-holes therein, for attachment of various electronic components therethrough. Such attachment is generally by means to assure good electrical conductivity. In the course of this, it has become desirable to provide for electrical conductivity from one surface through board through-holes to an opposite surface of the board. To this end, it is desirable to coat the through-holes with a metallic coating, preferably copper. Due to the nature of construction of most printed circuit boards; i.e., being a laminate, comprising a sandwich of generally a central core of plastic resin impregnated with fiberglass strands, in one or more layers, with exterior surface layers of copper sheeting, it has been necessary to find ways to connect the copper sheeting layers on the surfaces of the printed circuit boards, one to the other, and this is preferably done by coating the through-holes. It is known to provide a chemical preparation to the through-holes so that they will more readily accept copper applied thereto.
As industrial operations generate the need for more rapid production of printed circuit boards, processes have been developed for the continuous, or substantially continuous production of printed circuit boards. For example, in U.S. Pat. No. 4,015,706, a process is disclosed for treatment of printed circuit boards as they are horizontally conveyed from one station to another, in horizontal orientation, and such an arrangement is satisfactory for most of the cleaning, rinsing, etching, drying, processes and the like.
However, the plating of copper onto printed circuit boards, and especially the electroless plating in instances in which circuit boards are otherwise being treated as they travel through various stations, has heretofore required a substantially longer treatment process than the other treatments mentioned above, requiring substantially large apparatus, taking up large amounts of industrial manufacturing space, if the electroless plating operation is to be done as part of a continuous, or substantially continuous process.